The present invention is related to a dynamic polarity control method for driving a liquid crystal display (LCD), and more particularly to a content-based dynamic polarity control method for an LCD.
An LCD is constructed by an array of liquid crystal (LC) cells. FIG. 1 shows a schematic illustrating a panel structure of an LCD, wherein each LC cell 10 is coupled to a source driver 20, a gate driver 30 and a common voltage Vcom which is a reference voltage for the LCD. In FIG. 1, a timing controller (TCON) 40 controls the gate driver 30 to provide a gate voltage VG for turning on the LC cells 10 in each row line, and the timing controller 40 controls the source driver 20 to charge the LC cells 10 in each column line with a driving voltage VD. The gray level of a pixel or a dot indicated by the LC cell 10 is determined according to an absolute voltage difference between the driving voltage VD and the common voltage Vcom. Referring to FIG. 2, a relationship between the driving voltage VD corresponding to various gray levels and the common voltage Vcom is shown, wherein the polarity of the driving voltage VD can be either positive or negative when compared with the common voltage Vcom. For example, the signals V1(+) to V255(+) indicating the driving voltage VD with various voltage levels for gray levels 1 to 255, are larger than the common voltage Vcom, and the signals V1(−) to V255(−) indicating the driving voltage VD with the voltage levels for gray levels 1 to 255, are smaller than the common voltage Vcom. If most of the LC cells 10 are charged by the driving voltage VD with positive polarity, a positive voltage bias is induced in the common voltage Vcom, and vice versa. The voltage bias induced in the common voltage Vcom will cause the phenomenon of color shift and flicker. Thus, controlling the number of LC cells 10 driven by the driving voltage VD with positive polarity and negative polarity is important to keeping the common voltage Vcom at a neutral level.